Invariant NKT (iNKT) cells bridge innate and acquired immunity and play an important role in both protective and regulatory responses. The nature of the response is dictated by the initial cytokine environment: interaction with IL-10-producing cells induces negative regulatory T_h2/regulatory T cell-type iNKT cells, while that with IL-12-producing cells results in pro-inflammatory T_h1-type responses. Particularly, in the anti-tumor response, iNKT cells mediate adjuvant activity by their production of IFN-γ, which in turn activates both innate and acquired immune systems. Thus, upon activation of iNKT cells, both MHC⁻ and MHC⁺ tumor cells can be efficiently eliminated. On the basis of these mechanisms, iNKT cell-targeted adjuvant cell therapies have been developed and have shown great promise in initial clinical trials on cancer patients.